As an experienced technician, system builder, and ersatz engineer, I've known for many years that the speed of main memory (RAM) has little effect on overall system performance. Unfortunately while I've known this to be true, I never took the time to conduct my own tests to corroborate, and most of the tests I've seen posted online or in magazines have either been unscientific in nature or have tended to use overly optimistic synthetic benchmarks rather than real world applications - Until now.

Prompted by a question from one of my readers (thanks!), I was able to locate reliable data showing the true amount of overall system performance improvement one could expect from replacing slower memory with the same amount of faster memory: viewtopic.php?f=1&t=615

As you can see, the overall performance improvement is generally less than 1%. Why is this true? The answer is *cache*. More specifically I mean the L1/L2 (and sometimes L3) processor cache, which intercepts some 99% of the data transferred between the processor and RAM. In other words, due to the way the caches work, the processor normally accesses main memory directly only about 1% of the time.

Cache has been used in most PCs since 1986, which was the year PC processors running faster than 16MHz were introduced. It was at that point where the speed of the available DRAM (Dynamic RAM) could not keep up with the ever increasing processor speeds. A faster type of memory (Static RAM) was available, however it was much more expensive and far less dense, making it unsuitable for use as main memory. The solution was to use a small amount of the high-speed SRAM in the form of a cache closely coupled to the processor, thus insulating the processor from the slower main memory. For a more lengthy and detailed explanation (along with a very descriptive analogy), see the section "How Cache Works" on p. 73-78 in my book Upgrading and Repairing PCs 18th ed: http://www.amazon.com/gp/product/078973 ... elsupgr-20

Scott.

FascistNation

Post subject: Re: Is faster memory worthwhile?

Posted: Sun Aug 10, 2008 7:16 am

Joined: Sun Jul 20, 2008 10:50 amPosts: 508Location: Phoenix, AZ, USA

So are there any kinds of programs possibly being run where the L1/L2 would be exceeded and memory speed and arrangement (dual-channel) would come into play? ;-)

Scott

Post subject: Re: Is faster memory worthwhile?

Posted: Sun Aug 10, 2008 7:51 am

Site Admin

Joined: Sun Feb 04, 2007 11:44 amPosts: 6139

Is it possible to write a program that circumvents the cache? Sure, it is even possible to turn off the cache entirely, which is what any good memory diagnostics must do, if it wants to test the actual memory that is. Besides memory diagnostics, synthetic benchmarks specifically designed to test memory speed are a good example as well. Scott.

FascistNation

Post subject: Re: Is faster memory worthwhile?

Posted: Sun Aug 10, 2008 9:07 am

Joined: Sun Jul 20, 2008 10:50 amPosts: 508Location: Phoenix, AZ, USA

What I was getting at are there any circumstances you can envision in which the setups that people are running might actually cause the memory speed to be important?

Because it looks to me like you are thinking "no."

Scott

Post subject: Re: Is faster memory worthwhile?

Posted: Sun Aug 10, 2008 9:23 am

Site Admin

Joined: Sun Feb 04, 2007 11:44 amPosts: 6139

Quote:

...are there any circumstances you can envision in which the setups that people are running might actually cause the memory speed to be important?

Winning a benchmarking contest? <g> Scott.

twosteppin_ed

Post subject: Re: Is faster memory worthwhile?

Posted: Wed Aug 20, 2008 12:50 pm

Joined: Sat Apr 26, 2008 12:14 amPosts: 64Location: Port Orchard, WA

Hi Scott,

What I'm reading here is that the reason that substantial increases in memory speed don't translate into any material overall performance increases is that the CPU's memory cache intercepts most of the I/O between the CPU and the memory sticks.

My Q6700 has an 8MB L2 cache. The BOINC scentific work units that I crunch on this machine routinely use over *2GB* of memory when all 4 cores are running. This would indicate that the cache holds only a tiny, tiny fraction of the memory in use, resulting in the majority of the memory I/O traffic actually going out to the memory sticks because it is not in the cache.

In this case, woudn't there be a material increase in overall throughput and performance with faster memory, or am i still missing the picture here?

Thanks,Ed

Scott

Post subject: Re: Is faster memory worthwhile?

Posted: Wed Aug 20, 2008 1:17 pm

Site Admin

Joined: Sun Feb 04, 2007 11:44 amPosts: 6139

It really doesn't matter how much memory a particular program uses, the cache anticipates the specific memory addresses that will be requested in the near future, and retrives that data *in advance* of the program's actual request. When the program does request the data, the cache steps in and says "here you go", with no waiting for the actual RAM.

Thanks. That sounds interesting and would like to read how that works but your link isn't working. This is what I get:

"Looking for something? We're sorry. The Web address you entered is not a functioning page on our site

Go to Amazon.com's Home Page "

Thanks,Ed

ElBurro

Post subject: Re: Is faster memory worthwhile?

Posted: Tue Sep 09, 2008 8:00 pm

Joined: Thu Sep 04, 2008 7:30 pmPosts: 80

I thought everybody would find this article very interesting. Testing different speeds and sizes of memory but more based on a gaming setup. It basically agrees with what you suggested Scott that faster memory has little effect on overall system speed although amount of memory has a greater impact. However one interesting note is that testing Crysis on low detail going from 2 gigs of DDR2 667 to 2 gigs of DDR3 1600 did actually provide a 18% speed increase measured in frames per second. Of course those results need to be taken with a grain of salt since both sets of 1066 ram (DDR2 and DDR3) beat the 1600 rated memory.

That article does indeed offer further confirmation (thanks for posting the link). It also points out why I generally ignore the results of synthetic benchmarks, as they can overly exaggerate very small differences that don't mean much in the real world. Scott.

ElBurro

Post subject: Re: Is faster memory worthwhile?

Posted: Fri Dec 05, 2008 10:21 am

Joined: Thu Sep 04, 2008 7:30 pmPosts: 80

I know we've gone over the whole faster memory is not worth it for a while now but I just can't help myself when I read this article over at anandtech

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